We need more information, take a min or two to really try to tell someone else how to draw the mushroom with their back to the aquarium while you describe it to them (a picture here would
literally be worth a thousand words) There are quite a few corallimorphs that fit this description, usually from more temperate latitudes, and usually not autotrophic. You prolly ought to feed them some each week to make them grow, if they eat brine shrimp, then they are prolly feeders and based on your description, prolly do not have zooxanthellae. It could be that they have expelled their zooxanthellae as well, but without more info, we would all be shooting in the dark. Describe especially the margin of the capitulum, any tentacles appearing along the fringe, how pronounced the oral opening is, and whether th surface of the capitulum is smooth, rough, dempled, striated, bumpy, or hairy, and whether the irregularities follow a particular pattern. It would help to know the area it was collected from. Even then, the taxonomy of these creatures is not well developed, Aquarists are probably the best source of documentation and description of this class of creatures. (the literature is really thin, there could be undescribed spp. in 1 out of 100 tanks in the US alone...) I have a colony of 4 polyps of a similar spp in my prop system now, I have schedules it for propagation in about 2 more weeks (waiting for them to get to the 4 cm size before I cut them) I wish I could help you get a better idea of what you have, but there isn't much to go on, and even if you had pix, there isn't a lot of documentation to use anyway. Corallimorphs just don't seem to generate much interest in the marine biology community. Sea slugs , on the other hand...
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